This invention is in the field of apparatus and methods for performing Chemical Vapor Deposition (CVD), and relates more particularly to Atomic Layer Deposition (ALD) Processes.
In the field of thin film technology requirements for thinner deposition layers, better uniformity over increasingly larger area substrates, larger production yields, and higher productivity have been, and still are, driving forces behind emerging technologies developed by equipment manufactures for coating substrates in the manufacturing of various semiconductor devices. For example, process control and uniform film deposition achieved in the production of a microprocessor directly effect clock frequencies that can be achieved. These same factors in combination with new materials also dictate higher packing densities for memories that are available on a single chip or device. As these devices become smaller, the need for greater uniformity and process control regarding layer thickness rises dramatically.
Various technologies well known in the art exist for applying thin films to substrates or other substrates in manufacturing steps for integrated circuits (ICs). Among the more established technologies available for applying thin films, Chemical Vapor Deposition (CVD) and a variation known as Rapid Thermal Chemical Vapor Deposition (RTCVD) are often-used, commercialized processes. Atomic Layer Deposition (ALD), a variant of CVD, is a relatively new technology now emerging as a potentially superior method for achieving uniformity, excellent step coverage, and transparency to substrate size. ALD however, exhibits a generally lower deposition rate (typically about 100 Axc2x0/min) than CVD and RTCVD (typically about 1000 Axc2x0/min).
Both CVD and RTCVD are flux-dependent applications requiring specific and uniform substrate temperature and precursors (chemical species) to be in a state of uniformity in the process chamber in order to produce a desired layer of uniform thickness on a substrate surface. These requirements becomes more critical as substrate size increases, creating a need for more complexity in chamber design and gas flow technique to maintain adequate uniformity. For example, a 75 mm substrate processed in a reactor chamber would require less process control relative to gas flow, uniform heat, and precursor distribution than a 200 mm substrate would require with the same system; and substrate size is going to 300 mm dia., and 400 mm. dia is on the horizon.
Another problem in CVD coating, wherein reactants and the products of reaction coexist in a close proximity to the deposition surface, is the probability of inclusion of reaction products and other contaminants in each deposited layer. Also reactant utilization efficiency is low in CVD, and is adversely affected by decreasing chamber pressure. Still further, highly reactive precursor molecules contribute to homogeneous gas phase reactions that can produce unwanted particles which are detrimental to film quality.
Companies employing the RTCVD process and manufacturers of RTCVD equipment have attempted to address these problems by introducing the concept of Limited Reaction Processing (LRP) wherein a single substrate is positioned in a reaction chamber and then rapidly heated with the aid of a suitable radiative source to deposit thin films. Rapid heating acts as a reactive switch and offers a much higher degree of control regarding thickness of films than is possible with some other processes. RTCVD offers advantages over CVD as well in shorter process times, generally lower process costs, and improved process control. At the time of the present patent application RTCVD is a promising new technique for deposition of ultra-thin and uniform films. RTCVD is being steadily introduced into the commercial arena from the RandD stages by a number of equipment manufactures.
Although RTCVD has some clear advantages over general CVD, there are inherent problems with this technology as well, such as the temperatures that are used in processing. Larger surfaces require more critically-controlled temperature, which, if not achieved, can result in warpage or dislocations in the substrate. Also, the challenge of providing a suitable chamber that is contaminant-free and able to withstand high vacuum along with rapid temperature change becomes more critical with larger surface area requirements.
Yet another critical area of thin film technology is the ability of a system to provide a high degree of uniformity and thickness control over a complex topology inherent in many devices. This phenomena is typically referred to as step coverage. In the case of CVD, step-coverage is better than in line-of-sight physical vapor deposition (PVD) processes, but, in initial stages of deposition there can be non-preferential, simultaneous adsorption of a variety of reactive molecules leading to discrete nucleation. The nucleated areas (islands) continue to grow laterally and vertically and eventually coalesce to form a continuous film. In the initial stages of deposition such a film is discontinuous. Other factors, such as mean free path of molecules, critical topological dimensions, and precursor reactivity further complicate processing making it inherently difficult to obtain a high degree of uniformity with adequate step coverage over complex topology for ultra-thin films deposited via CVD. RTCVD has not been demonstrated to be materially better than convention CVD in step coverage.
ALD, although a slower process than CVD or RTCVD, demonstrates a remarkable ability to maintain ultra-uniform thin deposition layers over complex topology. This is at least partially because ALD is not flux dependent as described earlier with regards to CVD and RTCVD. This flux-independent nature of ALD allows processing at lower temperatures than with conventional CVD and RTCVD processes.
ALD processes proceed by chemisorption at the deposition surface of the substrate. The technology of ALD is based on concepts of Atomic Layer Epitaxy (ALE) developed in the early 1980s for growing of polycrystalline and amorphous films of ZnS and dielectric oxides for electroluminescent display devices. The technique of ALD is based on the principle of the formation of a saturated monolayer of reactive precursor molecules by chemisorption. In ALD appropriate reactive precursors are alternately pulsed into a deposition chamber. Each injection of a reactive precursor is separated by an inert gas purge. Each precursor injection provides a new atomic layer additive to previous deposited layers to form a uniform layer of solid film The cycle is repeated to form the desired film thickness.
A good reference work in the field of Atomic Layer Epitaxy, which provides a discussion of the underlying concepts incorporated in ALD, is Chapter 14, written by Tuomo Suntola, of the Handbook of Crystal Growth, Vol. 3, edited by D. T. J. Hurle, (copyright) 1994 by Elsevier Science B. V. The Chapter tittle is xe2x80x9cAtomic Layer Epitaxyxe2x80x9d. This reference is incorporated herein by reference as background information.
To further illustrate the general concepts of ALD, attention is directed to FIG. 1a and FIG. 1b herein. FIG. 1a represents a cross section of a substrate surface at an early stage in an ALD process for forming a film of materials A and B, which in this example may be considered elemental materials. FIG. 1a shows a substrate which may be a substrate in a stage of fabrication of integrated circuits. A solid layer of element A is formed over the initial substrate surface. Over the A layer a layer of element B is applied, and, in the stage of processing shown, there is a top layer of a ligand y. The layers are provided on the substrate surface by alternatively pulsing a first precursor gas Ax and a second precursor gas By into the region of the surface. Between precursor pulses the process region is exhausted and a pulse of purge gas is injected.
FIG. 1b shows one complete cycle in the alternate pulse processing used to provide the AB solid material in this example. In a cycle a first pulse of gas Ax is made followed by a transition time of no gas input. There is then an intermediate pulse of the purge gas, followed by another transition. Gas By is then pulsed, a transition time follows, and then a purge pulse again. One cycle then incorporates one pulse of Ax and one pulse of BY, each precursor pulse separated by a purge gas pulse.
As described briefly above, ALD proceeds by chemisorption. The initial substrate presents a surface of an active ligand to the process region. The first gas pulse, in this case Ax, results in a layer of A and a surface of ligand x. After purge, By is pulsed into the reaction region. The y ligand reacts with the x ligand, releasing xy, and leaving a surface of y, as shown in FIG. 1a. The process proceeds cycle after cycle, with each cycle taking about 1 second in this example.
The unique mechanism of film formation provided by ALD offers several advantages over previously discussed technologies. One advantage derives from the flux-independent nature of ALD contributing to transparency of substrate size and simplicity of reactor design and operation. For example, a 200 mm substrate will receive a uniform layer equal in thickness to one deposited on a 100 mm substrate processed in the same reactor chamber due to the self-limiting chemisorption phenomena described above. Further, the area of deposition is largely independent of the amount of precursor delivered, once a saturated monolayer is formed. This allows for a simple reactor design. Further still, gas dynamics play a relatively minor role in the ALD process, which eases design restrictions. Another distinct advantage of the ALD process is avoidance of high reactivity of precursors toward one-another, since chemical species are injected independently into an ALD reactor, rather than together. High reactivity, while troublesome in CVD, is exploited to an advantage in ALD. This high reactivity enables lower reaction temperatures and simplifies process chemistry development. Yet another distinct advantage is that surface reaction by chemisorption contributes to a near-perfect step coverage over complex topography.
Even though ALD is widely presumed to have the above-described advantages for film deposition, ALD has not yet been adapted to commercial processes in an acceptable way. The reasons have mostly to do with system aspects and architecture. For example, many beginning developments in ALD systems are taking a batch processor approach. This is largely because ALD has an inherently lower deposition rate than competing processes such as CVD and RTCVD. By processing several substrates at the same time (in parallel) in a batch reaction chamber, throughput can be increased.
Unfortunately, batch processing has some inherent disadvantages as well, and addressing the throughput limitations of ALD by batch processing seems to trade one set of problems for another. For example, in batch processor systems cross contamination of substrates in a batch reactor from substrate to substrate and batch-to-batch poses a significant problem. Batch processing also inhibits process control, process repeatability from substrate to substrate and batch to batch, and precludes solutions for backside deposition. All of these factors severely affect overall system maintenance, yield, reliability, and therefore net throughput and productivity. At the time of this patent application, no solutions are known in the industry to correct these problems associated with ALD technology as it applies to commercial production.
What is clearly needed is a unique and innovative high productivity ALD system architecture and gas delivery system allowing multiple substrates to be processed while still providing attractive throughput and yield, and at the same time using expensive clean-room and associated production floor space conservatively. The present invention teaches a system approach that will effectively address and overcome the current limitations of ALD technology, leading to commercial viability for ALD systems.
In a preferred embodiment of the present invention an ALD processing station for a cluster tool system is provided, comprising a processing chamber portion having a lower extremity with a first cross-sectional area; a base chamber portion below the processing chamber portion, the base chamber portion having a vacuum pumping port and a substrate transfer port, and a second cross-sectional area below the circular lower extremity of the processing chamber and the vacuum pumping port greater than the first cross-sectional area; a substrate support pedestal having an upper substrate support surface with a third cross-sectional area less than the first cross-sectional area and adapted to the base chamber portion below the transfer port by a dynamic vacuum seal allowing vertical translation; a vertical-translation drive system adapted to translate the substrate support pedestal to place the upper support surface at a processing position substantially even with the lower extremity of the processing chamber, or at a lower transfer position in the base chamber portion above the pumping port and below the transfer port; and a demountable gas supply lid mounted to the processing chamber, the lid for providing gases according to an atomic layer deposition (ALD) protocol. With the substrate support pedestal at the processing position the cross-sectional area of the substrate support pedestal and the larger first cross-sectional are of the form a first pumping passage having a first total effective area determining a fist limited pumping speed from the processing chamber portion through the vacuum pumping port, and with the substrate support pedestal at the lower transfer position, the cross-sectional area of the substrate support pedestal and the larger second cross-sectional area form a second annular pumping passage having a second effective area greater area than the first effective area, allowing a second pumping speed from the processing chamber greater than the first limited pumping speed.
In some embodiments the first cross-sectional area is formed by a replaceable ring, thereby allowing the first pumping speed to be incrementally varied by interchanging replaceable rings having constant outer diameter and differing inner diameter. There may also be an annular shroud surrounding a portion of the substrate pedestal beginning at the upper support surface and extending below the upper support surface, wherein the pumping area of the annular shroud at the height of the upper support surface is substantially equal to the first cross sectional area, such that, with the substrate support pedestal in the processing position the annular shroud mates with the first cross-sectional area constraining all gas flow from the processing chamber to flow within the annular shroud between the annular shroud and the substrate support pedestal.
In preferred embodiments the demountable lid closing an upper extremity of the processing chamber is mounted with a demountable seal, such that the lid and the dynamic vacuum seal may be demounted, allowing the substrate support pedestal to be withdrawn from within the base chamber region upward through the processing chamber region. The demountable lid in preferred embodiments comprises a gas distribution system for providing processing gases evenly over an exposed surface of a substrate supported on the substrate support pedestal with the substrate support pedestal in the processing position.
In some cases the substrate support pedestal comprises a closure plate parallel with the upper support surface and forms a vacuum boundary for the processing chamber, a heater plate on the processing chamber side thermally-insulated from the closure plate, and an electrically-isolated susceptor spaced-apart from and above the heater plate, the susceptor forming the upper support surface. The heater plate may be a composite heater plate having at least two separately-powered heating regions, allowing temperature profile across the plate to be managed by managing power to the separately-powered regions. In these aspects the inner heating region is separated from the outer heating region by at least one groove substantially through the heater plate. In a preferred embodiment the inner heating region has a cross-sectional are substantially equal to the cross-sectional area of a substrate to be heated by the heater plate. In some preferred cases the dynamic vacuum seal is a stainless steel bellows.
The present invention in its various embodiments provides a flexible and effective way t accomplish ALD processing on semiconductor wafers, and the various aspects of the invention are taught below in enabling detail.